Currently available devices and methods for measuring gas flow where surges in flow rate occur are notoriously inaccurate. This is particularly true where these devices and methods are used in measuring the discharge of accumulators in gas lift wells. A gas lift well is a type of well that brings oil and natural gas to the surface by injecting a catalyst gas into the well. The catalyst gas forces "pockets" of the oil and natural gas to the surface. Because the "pockets" of natural gas and oil brought to the surface vary in size and pressure, the gas extracted from these pockets in the accumulator exit the accumulator at varying pressures and thus have varying flow rates. Conventional devices, typically orifice meters, which are quite accurate within a certain range, have extreme difficulty in measuring the flow rate of the gas when it exits the accumulator at high velocities.
A typical orifice meter measures gas flow rate by measuring differential pressures across an orifice plate disposed within the gas stream. These differential pressures are directly proportional to the flow rates and are recorded on a circle chart recorder. If the flow rate surges to a point where it creates a differential pressure greater than the value which the chart recorder can measure, the recorder "pegs out" and cannot record the higher differential pressure value. Flow rate that occurs above the maximum value, therefore, cannot be recorded.
Typical orifice meters are placed in the pipeline at a location where it is desired to measure the gas flow. These meters have a circular cross-section which is axially aligned with the axis of the pipeline. This circular cross-section is concentrically reduced along the bore of the pipe. An orifice plate is disposed within the circular cross-section along the bore. As the gas flows through the reduced cross-section of the meter, a pressure drop is created from the upstream side to the downstream side of the orifice plate. The flow rate is proportional to the square root of the pressure drop, but there are a number of coefficients that have to be taken in account in defining the exact relationship.
A potential drawback of these known orifice meters is that they are only accurate over about a 3:1 turndown, i.e., the devices are only accurate when the maximum flow rate is no more than 3 times the minimum flow rate. The orifice of these devices is generally sized to fall within this range and produce the differential pressure that will drive the chart recorder over its normal range. So long as flow rates are within this range, the typical orifice meter produces accurate readings. Accuracy is lost if the meter is called upon to operate outside these limits, regardless of whether the readings are recorded onto a chart or input into some other readout device.
The present invention is directed to overcoming or at least minimizing some of the problems mentioned above.